1. Field of the Invention
The present invention relates to an image forming apparatus of the type in which an image formed in an image forming station is transferred to an image receiving member which is fed by feeding means to the image forming station, whereby an image is obtained on the image receiving member. More particularly, the present invention is concerned with an image forming apparatus suitable for use in the formation of a color image, by causing the image receiving member to pass through a plurality of image forming stations so that images of different colors are superposed on the image receiving member so as to form the color image on the image receiving member. It is to be noted, however, that the invention can be applied to various types of apparatuses, although a color image forming apparatus is a preferred embodiment of the invention and is disclosed in the specification.
2. Description of the Related Art
In general, a known image forming apparatus relying upon an electrophotographic process has an image forming station in which a latent image corresponding to image information is formed on an image forming medium such as an electrophotographic material, developing means in which the lateral image is developed to become a visible image, feeding means for feeding an image receiving member to the image forming station so as to enable the image on the image forming medium to be transferred to the image receiving member, and fixing means for fixing the image on the image receiving member.
The feeding means used in this type of image forming apparatus is generally classified into two types: a first type which comprises pairs of rollers disposed in the path of the image receiving member and adapted to nip the image receiving member therebetween, and a second type in which the image receiving member is placed on and fed by belt-type feeding members which extend between an inlet for the image receving member and the inlet of the fixing device. The feeding means of the first type is disadvantageous in that the end of the image receiving members tends to be damaged because it is repeatedly pinched by the successive pairs of rollers. Thus, the first type of the feeding means is unsatisfactory when considering feeding speed and adaptability of a variety of image receiving members. In addition, the length of the image receiving member that can be fed by this feeding means is undesirably limited by the span or spacing of the roller pairs, thus posing a restriction in the size of the image receiving member.
The second type of feeding means employs belt-type feeding members (conveyor belts) which feed the image receiving member from a pick-up position to an image transfer position where the image is to be transferred from the image forming medium, and thence to a fixing device. The belt-type feeding member than continues back to the pick-up position thereby defining a closed path. This type of feeding device is advantageous in that the risk for erroneously or wrongly feeding the image receiving member is remarkably suppressed as compared with the first type of the feeding means by virtue of the fact that the image receiving member is stably held on the feeding belts which run between the image receiving member inlet and the inlet of the fixing device. It is also to be noted that the first type of feeding means relying upon the roller pairs suffers from problems such as electrostatic charging of the image receiving member because of the presence of many sliding contacts between the image receiving member and stationary parts of the image forming apparatus. The sliding contact also poses a problem in that paper dust tends to be generated when the image receiving member is a sheet of paper. The second type of the feeding means relying upon feeding belts is free from these problems.
The second type of feeding means, i.e., the belt-type feeding means, however, encounters the following problems, though it offers the above-described advantages. One of these problems pertains to the contamination of the surface of the feeding belts, while another pertains to difficulty in recovering the normal state of operation in the event of a feeding failure. The contamination of the surfaces of the feeding belts causes problems such as contamination of the reverse side of the image receiving member and reduction in the efficiency of the transfer of the image from the surface of the image forming medium to the surface of the image receiving member. In most cases, the contamination of the surfaces of the feeding belts is attributable to deposition of toner particles, mainly the toner particles which have fallen from the developing means at these position above the feeding belt, but also from toner particles which have been transferred from the surface of the image forming medium to the belt surfaces. The heaviest contamination of the surfaces of the feeding belts is caused when an image receiving member which has jammed or failed to be fed properly is removed. In such a case, the portion of the image which still remains on the photosensitive member is transferred directly to the surfaces of the feeding belts.
For these reasons, feeding means which incorporate the feeding belts generally require a feeding belt cleaning device for the purpose of removing toner particles from the surfaces of the feeding belts, thereby to clean the surfaces of the belts. The cleaning of the surfaces of the belts, however, is generally difficult due to the fact that the feeding belts have been charged by a transfer charger, particularly when a large quantity of the toner particles has been deposited on the surfaces of the belts as in the case of the direct transfer of the toner particles from the image forming medium. It is, therefore, necessary to pass the contaminated portions of the feeding belts a plurality of times through the belt cleaning device.
The other problem, i.e., the difficulty in removing an image receiving member that has jammed or failed to be properly fed is attributable to the fact that, for the purpose of removing the image receiving member, it is necessary to lift the feeding belt mechanism which is heavier and larger than the other type of feeding mechanism, so as to create a space which is ample enough to allow the operator's hand to access the image receiving member. Thus, the operator is required to lift the heavy feeding mechanism.
Another problem encountered with the belt-type feeding means is that the separation of the image receiving member from the feeding belts is extremely difficult because the image receiving means is held on the belts by means of electrostatic attracting force. A mechanical and forcible separation may result in damaging the surfaces of the feeding belts.
Thus, the feeding means relying upon feeding belts inevitably encounters the above-mentioned problems, that is, the difficulty in the removal of the image receiving member which has jammed or failed to be properly fed and deterioration of the quality of the image due to contamination of the feeding belts, although this type of feeding means provides excellent feeding performance.
Referring now to the fixing means, there are broadly two types of fixing means: a heat-type fixing means which welds an image to the image receiving member by application of heat, and pressure-type fixing means which fixes the image by application of pressure. The heat-type fixing means provides much higher fixing performance and, therefore, is more popular than the pressure-type fixing means. Among various heat-type fixing devices, the most widely used is a fixing device of the type called heat-roller fixing device which makes use of a pair of heated rollers through the nip of which the image receiving member is passed so as to be heated to fix the image thereon, because this type of fixing device offers superior fixing effect and economical use of energy.
The fixing device of the heat-roller type, however, encounters the following problems. Namely, when a long image receiving member is used, the leading end of the image receiving member nipped between the heat rollers is vibrated by these rollers and the vibration propagates along the image receiving member to affect adversely the transfer of the image which is being conducted on a downstream portion of the image receiving member, resulting in a degradation of the image. Another problem is that the fixing performance is determined by the heat capacity of the image receiving member.
In order to obviate these problems, a known feeding device is designed such that the peripheral speed of the heat rollers is selected to be slightly lower than the speed of the image receiving member so that a loop of the image receiving member is formed between the fixing rollers and the feeding device so as to cut off the propagation of vibration, thus preventing degradation of the image quality. In order to overcome the problem concerning the heat capacity, attempts have been made to determine the rate of heat input so that it matches for the heat capacity of the image receving member most often used. When an image receiving member of a greater heat capacity is used, the feed speed of the heat rollers is reduced to increase the heat input to the image receving member per unit time, thus improving the fixing effect. This countermeasure, however, poses another problem. Namely, when an image receving member having a large heat capacity, e.g., a resin sheet, a post card, a cardboard or the like, is used in combination with the heat-roller type fixing device, a large difference is caused between the peripheral speed of the fixing rollers which are operating at a reduced speed and the speed of the feeding device. In particular, when the image receiving member is long, a large loop of the image receiving member if formed in the portion upstream of the fixing rollers so that the image receiving member interferes with the path of the image which has not been fixed yet, with the result that the toner image collapses or feeding fails due to propagation of the loop and due to the stiffness of the image receiving member. In order to overcome this problem, various measures have to be taken such as widening the inlet of the fixing device or providing a large space along the feed path of the image receiving member. Such measures, however, undesirably increase the size of the image forming apparatus and reduce the rigidity thereof.
Thus, problems still remain unsolved, even when the best feeding and fixing means are employed.